1. FIELD OF THE INVENTION
The present invention relates to rapidly tunable lasers in Littrow configuration, and particularly to pulsed lasers of this type.
2. PRIOR ART
Plane and concave reflective gratings in Littrow configuration are mounted so that the surface thereof intercepts the optical axis of a spectrograph or laser. In combination with a laser, the surface of a grating is placed at various angles to the optic axis so that different wavelengths of incident light will be reflected along the laser's optic axis. The particular wavelength .lambda. reflected is determined by the formula: EQU k.lambda.=a sin .theta. (1)
where a is the grating element spacing, .theta. is the angle of incidence of the laser light relative to the grating normal, and K is a whole number determined by the order employed.
Gratings mounted for rotation so that .theta. can be varied to thereby select various wavelengths and tune the laser, are disclosed in the prior art. Four U.S. patents provide examples of tunable lasers with Littrow gratings. In U.S. Pat. No. 3,443,243 to Patel, a grating is located beyond the reflective elements defining the optical resonator cavity of a laser and light reflected from the grating passes through an aperture to maximize frequency resolution. The concave reflective grating employed in Patel's device can be rotated about an axis parallel to the grating lines. In U.S. Pat. No. 3,739,295 to Shah, a rotatable plane reflective grating is employed as a tuning element in a dye laser. An aperture is included between the grating and one of the resonator cavity reflector elements to block fluorescence of radiation returning from the grating to the lasing medium. In a patent to Comera et al, U.S. Pat. No. 4,241,318, a laser's plane reflector grating is adapted, in combination with a wheel containing two optical elements, to place the optical elements periodically in the path of the laser beam. This deflects the laser beam so that the angle of incidence of the beam on the grating is modified and a different wavelength is reflected back along the longitudinal axis of the laser for each element. The grating and wheel are rotatable as a unit relative to a plane perpendicular to the longitudinal axis of the laser so that more than two wavelengths can be selected.
An embodiment shown in FIG. 9 of U.S. Pat No. 4,287,486 to Javan discloses a double grating arrangement with the gratings facing each other, albeit offset and not parallel, so different wavelengths of light from the laser are dispersed onto a mirror. The mirror is rotated to sequentially regenerate only one of a series of wavelengths at a time. The laser is triggered to fire when light of the first wavelength strikes the mirror in perpendicular relationship, with the pulse continuing until all the wavelengths of interest are scanned. Thus a chirped pulse (i.e. a pulse with a change in wavelength within the pulse) is provided. Javan however, does not disclose a pulsed laser wherein each pulse can be tuned to a different wavelength, particularly if very fast switching times are desired.
In "CO2 Probe Laser with Rapid Wavelength Switching", S. Holly and S. Aiken, SPIE Volume 122, Advances in Laser Engineering (1977), rapid tuning of a continuous wave CO2 probe laser is provided by positioning eight gratings in carousel fashion about a mirror mounted on a scanner, stepping motor apparatus. The eight gratings are switched in sequence into the optical cavity of the probe laser. Switching between wavelengths was reported to occur within approximately 10 milliseconds. The number of wavelengths which can be scanned by the Holly and Aiken device is limited by the number of gratings provided and the alignment problems require a complex electro optics control loop system.
Not found in the prior art is a relatively simple system for rapidlly scanning (i.e. on the order of 10 milliseconds or less) dozens or even one hundred wavelengths from a single laser source. Such a system would be particularly useful in spectroscopic measurements both in diagnostic laboratory experiments, remote sensing systems for pollutants and toxic gases, and in certain laser weapon systems.